Mechanical Fuel pump question !

General engine tech -- Drag Racing to Circle Track

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Mechanical Fuel pump question !

Post by SBC-68-FIREBIRD » Sun Jul 23, 2017 11:14 pm

Hi Guys,

My 421SBC is a pretty mild build & made 580HP/550TQ. On the dyno the Edelbrock 1711 ( 130GPH ) mechanical pump kept up okay.
When I put the engine in the car which should be making at least 50HP more than the old engine it ran exactly the same ET & MPH @ the track which started me looking for reasons why.
I first changed the converter from 3800 to 5000 which it only picked up half a tenth in the 60' & that was it.

I then ran a temporary line from just near the carby & took it for a run on the street. Just driving around fuel pressure sits on 7 - 7.5psi & even with a hit in top gear no problems. From a standing start under full load by the time you click into second the pressure is dropping right down to about 3psi but then its time to back off.
I was running the original 3/8" pickup & line to the pump.
Fitted a 1/2" pickup into the tank & ran 1/2" hardline from tank to pump & onto carby & it does exactly the same thing.

I don't feel the car slowing down at the track like its running out of fuel ( it runs 10.9@122mph ).

This pump is rated at 130GPH, can support 600HP, has 3/8" inlet & outlet ports, pumps out 10psi.

To me it is looking like the pump is not up to the job. It may be okay on a dyno but not in the car.

I am trying to avoid going to an electric pump but don't want to spend the money on a Clay Smith or RobbMC mechanical if I am going to have the same issue. Anybody had the same sort of issues or any idea's on a mechanical pump that WILL work ?


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Re: Mechanical Fuel pump question !

Post by cgarb » Mon Jul 24, 2017 12:00 am

I like my C&S heavy duty 6 valve pump. It says its rated 240 gph...honestly it has the same valves in it as my Barry grant pump. Not sure where the extra flow is coming from but it looks like a quality unit for sure. I'm feeding a 377 ci methanol carb sbc that is 560ish HP. No problems. I have 10an feed to the pump and 8an lines to the carb, using a throttle bypass and an 8an return. Clay Smiths are nice pumps also.

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Re: Mechanical Fuel pump question !

Post by Geoff2 » Mon Jul 24, 2017 5:39 am

600 hp will require about 50 gph, assuming correct A/F ratio. Allowing for a 50% safety margin, 75 gph delivered to the carb is plenty.

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Re: Mechanical Fuel pump question !

Post by cjperformance » Mon Jul 24, 2017 7:51 am

Rubber line on the suction side collapsing?

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Re: Mechanical Fuel pump question !

Post by SBC-68-FIREBIRD » Mon Jul 24, 2017 9:32 am

The suction side is 1/2" hardline from pickup to within 6"of pump so all okay there.

The pump either cannot suck enough when the car is under full acceleration or cannot recover quick enough once the pressure drops after the pump.

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Re: Mechanical Fuel pump question !

Post by F-BIRD'88 » Mon Jul 24, 2017 11:18 am

If the pump is holding a steady 3psi fuel pressure when flowing WOT
and the car's 1/8th MPH and 1/4 mile MPH agree A X 1.23 ='s B
then the pump is not the limiting thing.

if you want to test to see if acceleration G force is effecting fuel flow
you can temp install a fuel cell under hood of the car and run it
down the 1/4. Now gravity G force is helping fuel flow.

You could also install a simple AFR gauge/O2 sensor on the exhaust.
if the fuel flow (volume not pressure) to the carbs is being starved by G force the AFR will drift towards "leaner afr" as you progress thru a drag pass reguardless of how rich the carb jetting is set. This AFR gauge need not be a wide band type. a simple cheap marrow band is fine to detect a AFR drift fuel flow problem. if the AFR gauge holds steady all the way and thru the 1/4 mile traps it is not the fuel pump nor G force.

The actual WOT AFR reading on the AFR gauge is not the critical thing. you are looking for a progressive AFR DRIFT towards leaner AFR from "normal WOT AFR" not absolute numbers.
if the pump is not doing its job it will CLEARLY show up on the AFR gauge (narrow or wide band equally well).

Actual acting G force is only really momentarily high during initial launch and thru first gear acceleration. The G force drops off a lot after initial launch.
But what can effect the perf of a mechanical fuel pump on a drag car is HEAT.
HEAT (Hot soak) is what can effect a mechanical fuel pump on a drag car...
If the feed side and or the fuel pump get too hot at the track,
then the fuel tends to vapour lock. The mech pump cannot move
fuel if it is not in a liquid state. Bubbles (vapourized fuel) stop fuel pump flow. "Vapour lock"

Keep the feed side fuel lines away from hot exhaust/HEADERS. The mech fuel pump
may need a **SEALS IT** heat isolator mounting gasket/HEAT shield between it and the mounting flange on the engine to stop excess fuel pump body temp.
This is what messes with mechanical fuel pumps specifiably used for drag racing.
The fuel pump and under body fuel feed side fuel line tend to "Hot soak" in the staging lanes.
If either get too hot fuel delivery is inconsistent. You usually see this problem at the track
on mechanical fuel pump drag cars as a sag or bog in performance at the top of first gear, then recovery.
If you cannot eliminate the pump/fuel line HOT SOAK, adding a auxiliary "pusher pump" at the back helps as it pushes the vapour bubbles infected fuel thru .

The slight 5psi ish pressure it applies to the feed side fuel line stops the fuel from vapourizing
at low liquid pressures as seen on the long length working feed side of a mechanical fuel pump
You don't need a huge "pusher pump" or huge fuel pressures (a Carter P4070 electric pump is plenty) and you only need turn it on if at the track drag racing.

On your feed side fuel pick up tube in the tank make sure that the end of the pick up tube is either bell flaired or angle slash cut a bit to max pickup flow.

Do you have a fuel pressure regulator between the pump and carb?
Is it a actual restriction?

Dyno test corrected HP VS real uncorrected engine HP:
We race in a real uncorrected world...

On your dyno test look at the power correction factor that was used to get the
"580 Horsepower". What is that correction factor? Now you can see the real uncorrected engine horsepower that the motor really made on the dyno that day.

Now compare the historical weather data for the day on the dyno and the day at the track.

Worse air? Better air at the track that day? or...
Look on the US national weather service web site for the historical weather data for that day and location. This will have a effect on actual real world engine power performance.

The corrected horsepower data from a dyno test is for comparing to other properly accurately corrected dyno tests. The Uncorrected raw dyno test power and torque data is what the engine is actually doing that day under those local weather station conditions.
Either get the dyno correction factor used on that dyno test or get the raw uncorrected power printout and compare.
When you are racing the car you are working with real raw uncorrected power
and track weather conditions effect that, including wind speed and direction.

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Re: Mechanical Fuel pump question !

Post by Tuner » Mon Jul 24, 2017 12:05 pm

Recently went through this same scenario with a '56 Chevy that has a recently installed, new, improved, swoopy cool, restoration fuel line and tank gauge sender unit with 3/8" steel line. Holley 110 GPH 8PSI pump.

At WOT the pressure dropped to 3PSI. Engine ran OK and A/F kept up with normal expectations down to 4 PSI, but below 4 the A/F fell off lean and engine surged. 400 SBC, sporty cam, headers, QJet, .135" inlet seat.

The fuel level sender unit with the fuel line exiting the tank has a female 3/8" inverted flare brass fitting soldered to the line coming out of the tank and the China solder job was sucking air. It is above fuel level so it didn't leak, though it was moist.

Spent too much time trying to fix the carburetor that didn't have anything wrong with it except it always ran the same ET like clockwork, like it was stuck or something, and only gets 18 MPG.

When the fuel changed in the early '70s the cars with rubber line connecting the tank outlet on top of the tank to the steel line had a lot of trouble with this. The line would crack and suck air but not leak fuel.

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Re: Mechanical Fuel pump question !

Post by Bradley67 » Mon Jul 24, 2017 2:00 pm

If running an unvented fuel tank cap check the fuel tank vent line. It may be blocked or partially clogged. On larger demand systems sometimes multiple vents need to be installed in the fuel tank.
Also check every connection for possible air leak. Those are difficult to find but can cause problems just like the issue your having.

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Re: Mechanical Fuel pump question !

Post by SBC-68-FIREBIRD » Fri Jul 28, 2017 10:22 am

Thanks for the replies guys.

I have replaced from the pickup to the pump with 1/2" hardline with about 8" of rubber at the pump.
The pump has 3/8" inlet & outlet ports.
I then ran 1/2" to my Mallory filter.
1/2" out of filter to 12-803 Holley reg @ carby.
Still losing pressure under hard acceleration.

Changed the 1/2" lines after the pump back to 3/8" as the pump only has a 3/8" outlet & thought there may be too much volume in the bigger pipe. Still losing pressure.

I already had an Edelbrock 1727 reg so fitted that & still doing the same thing.

Has anybody had any experiences using one of these Edelbrock 1711 - 130GPH fuel pumps ?

Am still trying my guage before & after reg but its just taking time trying to fit in with work. Maybe I am better off with something like a Clay Smith pump that has 1/2" inlet & outets on the pump & puts out 14PSI so it has more of a buffer zone ?

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Re: Mechanical Fuel pump question !

Post by rfoll » Fri Jul 28, 2017 11:34 am

I'm not sure I read your flow path correctly, but generally you want the filter on the pressure side of the pump. The pressure side of a pump is vastly stronger than the weak action of atmospheric pressure feeding it.
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Re: Mechanical Fuel pump question !

Post by prairiehotrodder » Fri Jul 28, 2017 11:42 am

i've had better luck with the carter mechanical pumps. I had an edelbrock fail once. I would suggest a AEM "fail safe" AFR gage so you can data log a pass. they work pretty slick. I am like you and would prefer to stay away from electric pumps if possible.
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Re: Mechanical Fuel pump question !

Post by BOOT » Fri Jul 28, 2017 7:28 pm

I've had two carter fuel pumps and had issues since switching from a stock pump. Holley is on my list to try next.
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Re: Mechanical Fuel pump question !

Post by steve cowan » Sat Jul 29, 2017 5:10 pm


personally i think that pump is to small as in 3/8'' inlet/outlets for your application
you are making some good power and high tens in a streeter is plenty cool
my ride is a 3500 pound utility similar to elcomino but leafsprings
383 sbc approx 510 hp
car runs 11.1s @ 121.5 mph
full street trim,pump gas,255 drag radial drive to and from the track 3- 3 1/2 hr round trip,lots of street cruising
photos i put up are
60 ltr foam filled fuel cell -8 breather -8 return,1 x -10 outlet in use,1 x -8 outlet blocked
-10 braid to -8 ball valve fuel shutoff for ease of removing -10 aeromotive stainless filter
-10 entire length of vehicle inside of chassis rail
i am using a holley 200 gph mechanical fuelpump with a composite pushrod
-8 out of fuelpump to a QFT fuel log at the carb fitted with a aeromtive liquid filled guage
-8 out of log to a holley bypass regulator mounted on firewall with a -3 braid to a autometer fuelguage dampner and internal guage in car
-8 return to the top of the tank
the fuel cell is higher than the fuelpump,i think this is important
my car has only 10 passes on this new combo including the mechanical fuelpump deal as i have only used a electric pump in all my tough rides
previously,i dont hink anyone should overlook a decent fuel system in any application
my fuel pressure is rock steady all the time,the fuel pressure guage under the bonnet does flicker a bit due to pump pulse but the incar one is good
my air fuel readings stay at 13.1 for the entire run and plugs look sweet as well
fuel pressure does not move
i hope this might give some ideas for your car which by the way is mighty tough thank you very much
steve c
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Re: Mechanical Fuel pump question !

Post by SBC-68-FIREBIRD » Mon Jul 31, 2017 3:50 am

rfoll wrote:I'm not sure I read your flow path correctly, but generally you want the filter on the pressure side of the pump. The pressure side of a pump is vastly stronger than the weak action of atmospheric pressure feeding it.
The filter is definately on the pressure side of the pump.

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Re: Mechanical Fuel pump question !

Post by SBC-68-FIREBIRD » Mon Jul 31, 2017 4:01 am

Looks nice Steve.

I rang Rocket here in Sydney today regarding that same fuel pump you run - $820 :shock: There is no chance I will be paying that.

Am just very undecided which way to go -

The Carter 4601HP is supposed to be a very reliable electrical pump. Pumps out 100GPH & 15PSI. I would think that 100GPH should be plenty to feed this engine plus at 15PSI it should have some line pack to keep the pressure up on hard launches ?

I cant run a Clay Smith or RobbMC mechanical as they are too big. The Edelbrock 130 I have on it I cant get a finger between the pump & the cross member. Holley makes a 170GPH mechanical which has 1/2" inlet & outlets but only at 8PSI so under hard acceleration is that 8PSI going to drop off & give me the same issue I am having now ? If I knew the mechanical was going to be okay I wouldn't even hesitate but I don't want to buy a $250 Holley pump that doesn't fix the problem.
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